How do I move multiple vertices to the same point at the same time?
The new method, ALTM, has already been mentioned. The other solution is to set the pivot point in the header to either active or cursor and then scale the selection to zero S0RETURN. Scaling or rotating around the active element is pretty cool since it makes the last selected, white vertex the center.

It's actually useful to know both ways because with scaling you can exclude an axis. Let's say you want to move them all to the XY position of some topology but leave Z unchanged. Then you can place the cursor on the target and use SSHIFT-Z0RETURN. There's just the additional step of W4 remove doubles.